α1/α2-Adrenoceptor agonist selectivity of mono- and dihydroxy-2-N,N-di-n-propylaminotetralins

The pressor activities and the identity of the postjunctonal α-adrenoceptors involved were determined for a series of congeneric mono- and dihydroxy-substituted 2-N,N-di-n-propylaminotetralins and N,N-di-n-propyldopamine (DPDA) following i.v. administration to pithed normotensive rats. The affinity for α1- and α2-adrenoceptor-like binding sites was obtained from radioligand displacement studies. The 5- and 7-OH substituted tetralins as well as DPDA were reasonably potent and about equieffective pressor agents. The 6-OH congener had almost no vasoconstrictor effects whereas the 8-OH positional isomer occupied an intermediate position. The 5,6- and 6,7-di-OH analogs very effectively raised the diastolic pressure of pithed rats. On account of the inhibition exerted by prazosin (0.1 mg/kg) and yohimbine (1 mg/kg) the 5- and 7-OH isomers as well as DPDA can be classified as mixed α1/α2-adrenoceptor agonists, the α1-adrenoceptor-stimulating potency being more pronounced, especially for the 5-OH congener. In addition, a significant contribution of serotonin receptors to the pressor responses to the 8-OH compound was detected. Similarly, α2-adrenoceptors were mainly responsible for the vasoconstriction caused by the 6,7-di-OH isomer, whereas the 5,6-di-OH congener very selectively stimulated this α2-type receptor in the lower dose range and α1-adrenoceptor stimulation predominated at higher doses of this agonist. The 6,7-di-OH compound failed to activate vascular postjunctional β2-adrenoceptors. The results indicate that the α1/α2-adrenoceptor agonist activity depends on the position(s) and the number of hydroxy groups present as well as on the alkyl substitution at the amino function. 2-N,N-Di-n-propylamino-6,7-dihydroxytetralin may be a more suitable α2-adrenoceptor selective agonist than M-7.